The present invention relates to telomers of the formula ##STR2## where n is in the range of 1 to 10, and processes for preparing such telomers. The telomers of this invention are saturated, low molecular weight polymers which are useful for preparing non-flammable hydraulic fluids.
Various methods of preparing chlorotrifluoroethylene ("CTFE") telomers are known in the prior art and have been practiced commercially for many years. An article by William T. Miller, Jr. et al in Industrial and Engineering Chemistry, pages 333-337 (1947), entitled "Low Polymers of Chlorotrifluoroethylene", describes a process for producing low molecular weight polymers of CTFE by polymerization in a solution of chloroform using benzoyl peroxide as a polymerization promoter. Other solvents disclosed in the reference as being useful for this purpose include carbon tetrachloride and tetrachloroethylene. The solution is heated in a pressure vessel for 13/4 hours at 100.degree. C., and the unreacted CTFE monomer and chloroform are removed by distillation, leaving a crude telomer of general formula CHCl.sub.2 (CF.sub.2 CClF).sub.n Cl, which can be further heated and distilled to yield products ranging from a light oil to a semi-solid wax or grease.
Another process for preparing low molecular weight CTFE polymers is described in U.S. Pat. No. 2,788,375, issued Apr. 9, 1957. This process comprises reacting CTFE with a saturated brominated compound in the presence of a source of radiation. Suitable brominated compounds include 1,2-dibromo-2-chlorotri-fluoroethane (CF.sub.2 BrCClFBr). The saturated bromopolychlorofluoro compounds obtained by this process can then be distilled, and the isolated fractions reacted with chlorine to prepare polychlorofluoro compounds. The compounds are predominantly higher molecular weight telomers, i.e. n is greater than 4.
Czechoslovakian Pat. No. 201,708, published Aug. 15, 1982, discloses the reaction of CTFE with CBrClFCF.sub.2 Br using a source of radiation at a temperature of from 20.degree. C. to 30.degree. C. to prepare 1,4-dibromo-2,3-dichlorohexafluorobutane and 1,6-dibromo-2,3,5-trichlorononafluorohexane as principal reaction products. These compounds are designated by the following structural formulas: ##STR3## Both compounds (1) and (2) share the common feature of having the same end groups, i.e., --CF.sub.2 Br, as well as adjacent pairs of --CFCl-groups in the body of the telomer. Telomers having structures such as (1) and (2) are believed to be inherently more unstable and are less easily separated from impurities than the telomers of the present invention.
A more recent development in this field is described in a series of articles by Y. Pietrasanta et al entitled "Telomerization by Redox Catalysis" appearing in the European Polymer Journal, Vol. 12 (1976). This technology involves the reaction of single carbon halogenated telogens, such as CCl.sub.4 and CCl.sub.3 Br, with CTFE in the presence of benzoin and a suitable redox catalyst, such as ferric chloride. The telomerization reaction is suitably carried out in acetonitrile which is a common solvent for the reactants and catalysts. The telomerization reaction can be illustrated as follows: ##STR4## where X is chlorine or bromine. The reference further discloses that the use of CCl.sub.3 Br as a telogen results in a lower degree of telomerization and a higher proportion of monoaddition product than with the use of CCl.sub.4.
The redox process has the advantage of directly preparing low molecular weight products without the necessity of cracking or fractionating a higher molecular weight polymer.
A modification of the redox process is disclosed in commonly assigned European Patent Application No. 83302397.1, published Nov. 9, 1983. In this modification, nickel is substituted for benzoin in reaction (3). This process has the advantage of simplifying the reaction by eliminating the necessity for removing benzoin from the reaction mixture, whic is a time-consuming and costly operation. In addition, when telomers prepared using such a system are fluorinated, the resulting product does not contain unsatisfactory levels of unsaturation.
It is therefore a principal object of the present invention to provide a process for preparing novel CTFE telomers which can be readily separated into relatively pure isomers and which can be further chlorinated to prepare non-flammable hydraulic fluids.